Kinematically Complete Study of Dissociative Ionization of D2 by Ion ...

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(Received 30 November 2005; published 1 May 2006). We present a kinematically complete study of dissociative ionization of D2 by 13:6 MeV=u S15‡ ions.
PRL 96, 173201 (2006)

week ending 5 MAY 2006

PHYSICAL REVIEW LETTERS

Kinematically Complete Study of Dissociative Ionization of D2 by Ion Impact G. Laurent,1,2 J. Ferna´ndez,2 S. Legendre,1 M. Tarisien,1 L. Adoui,1 A. Cassimi,1 X. Fle´chard,3 F. Fre´mont,1 B. Gervais,1 E. Giglio,1 J. P. Grandin,1 and F. Martı´n2 1

Centre Interdisciplinaire de Recherche Ions Lasers (CIRIL)—CEA-CNRS-ENSICaen, rue Claude Bloch, BP 5133, F-14070 Caen cedex 5, France 2 Departamento de Quı´mica C-9, Universidad Auto´noma de Madrid, 28049 Madrid, Spain 3 LPC-Caen, 6 Bd du Mare´chal Juin, F-14050 Caen cedex, France (Received 30 November 2005; published 1 May 2006) We present a kinematically complete study of dissociative ionization of D2 by 13:6 MeV=u S15 ions. The experiment allows us to unravel the competing mechanisms, namely, direct single ionization, autoionization of doubly excited states, ionization excitation, and double ionization, and to analyze the corresponding electron angular distribution from fixed-in-space molecules. The conclusions are supported by theoretical calculations in which the correlated motion of all electrons and nuclei and the interferences between them are described from first principles. DOI: 10.1103/PhysRevLett.96.173201

PACS numbers: 34.50.Gb

Ionization of the simplest H2 molecule by the impact of photons [1–7], electrons [8–11], and ions [12 –18] has been the subject of extensive experimental and theoretical investigations for almost three decades. With the advent of kinematically complete collision experiments, in which the full momentum vector of all charged particles is determined, it is now possible to investigate these processes with unprecedented detail and precision. So far a few examples have been reported in the literature for photoionization [19–23] and electron impact ionization [24]. In contrast with photoionization, in which a well-defined amount of energy is absorbed by the target (the photon energy), the energy absorbed by H2 in a collision with a fast charged projectile follows a distribution with practically no upper limit. Thus all processes capable of ionizing the molecule are energetically allowed and can, in principle, occur simultaneously. It also means that ionization can involve one or all electrons of the molecule. In the case of ion impact, the only existing electron-ion coincidence experiment has been reported by Dimopoulou et al. [25]. In this work both nondissociative and dissociative ionization of H2 were investigated in the range of very low ion kinetic energy (8 eV. This is not surprising because this is the region where DI becomes important [Fig. 2(a)] and the corresponding channels are not included in the calculations. The agreement is also reasonable for the singly differential cross sections [Figs. 1(b) and 2(b)] except for very low electron energies. The analysis of the final state wave function allows us to separate the individual channels contributing to IE and AI. At D energies of 5 eV, ionization through the 2 u 2pu  threshold is the dominant process; however, its sole contribution is not enough to explain all features observed experimentally. Indeed, higher channels are responsible for the second maximum observed in Fig. 2(b). The discrepancy with experiment around this second maximum indicates that IE leads to highly excited D 2 states not included in the theory (namely, beyond the 11th D 2 state). Therefore one has to be very careful in trying to fit the total D distribution to an incoherent sum of FC factors associated with only a few IE channels. Although such a procedure may lead to DSI, IE, and DI distributions in qualitative agreement with experiment [26], it fails in predicting the correct contribution of each individual threshold. The linear behavior observed between 1 and 4 eV in Fig. 1 is mainly due to autoionization of the Q1 states and, to a lesser extent, of the Q2 ones. The Q3 and Q4 states play a minor role. At very low D energy (